As greenhouse gas generating from fossil fuel is pointed out and blame as an element causing global warming, United Nations Framework Convention on Climate Change was adopted to prevent global warming in a worldwide basis, and various programs are being implemented to contain these greenhouse gas emissions. Our country is also faced with the reality to find a solution to reduce greenhouse gas emissions and take various measures to reduce use of fossil energy.
For this reason, new renewable energy, which is defined as a new energy source replacing fossil fuel such as oil, coal, nuclear, natural gas, etc., is receiving recognition, and new renewable energy refers to, and incorporating, energy using new energy such as coal to liquid, hydrogen energy, etc. and animal, plants, organic matter, sunlight, wing, water, geothermal heat, etc. to transform to an eco-friendly and renewable energy.
These new & renewable energy has advantages of being renewable, eco-friendly, and unlimited, but has issues of continuous research and development for increase in efficiency and overcoming current uncertain market forecasts.
Wind power generation, which is one of the new renewable energy using wind, is a technology that uses aerodynamic characteristics of kinetic energy of air flow to rotate a rotor to transform to mechanical energy, and generates electrical energy from this mechanical energy.
Wind power generation field is an industry recently emerging as an alternative energy source and sustaining high growth, and growth is accelerating due to obligations for worldwide greenhouse gas reduction, lower power generation costs from technological advancements, etc.
Since wind power generation uses unlimited pollution free wind scattered everywhere, effects on environment is small, territory may be used efficiently, and in the case of power generation costs for large scale power plants, it is a very useful power generating method without decrease in efficiency compared to prior power generating methods.
However, since power generation is difficult when energy density is low due to lack of wind, it should be installed restricted to certain areas, and since power generation is only able when a required amount of wind exist, equipments such as storage devices, etc. are needed for stable supply of electricity, and problems of noise generating from recent enlargement of wind power generators exist, and disadvantages of high initial investment costs exist.
Structure of prior wind turbines are composed of a high-rise tower erected on the ground, a nacelle installed on top of the tower, and a rotation axis combined with the nacelle and a number of blades installed on the surface of the outer circumference of the rotation axis, and an acceleration unit, power generator, control device, etc. is placed inside the nacelle so that the torque of the blade reaches the power generator through the rotation axis.
In this instance, the blade generally forms a pitch angle of 30° with a hub section, where it is coupled with the rotation axis, and pitch angle of about a 2°˜3° with the tip section, and is formed in a twisting fashion and also in a shape where the width and thickness gradually becomes narrower and thinner. In this case, the blades rotate when a blades with a pitch angle receives wind blowing on the front slantly and naturally passes it through to the back of the rotating surface of the blade and generates drag, and at the same time lift is generated in the end section of the blade in respect to the rotation of the blade and thus torque of the blade is improved. That is, since energy is transferred to a blade when wind blowing to a rotating surface of a blade passes through a rotating surface, velocity of wind reduces to about ⅔, and thus maximum of 59.26% (Betz Law) of flow energy of wind converts to rotation power.
But, since wind turbines mentioned above have a structure transferring rotation power to a blade when wind passes through a rotating surface of a blade, wind velocity decreases after wind passes through a blade, and since amount of energy corresponding to the decreased velocity of flow energy of wind converts to torque, it is limited in that efficiency of power conversion (Cp) is not high. For this reason, it may be properly applied to large sized wind turbines with lower share of base loads, but with small sized generators, which do not, efficiency of power conversion (Cp) relatively falls below about 30% and problems of decreased power generation output arise.
Also, prior wind turbines possess problems of decreased efficiency of usage in low wind velocity domains because required starting wind velocity is high due to structural characteristics of a blade. Specifically, taking into account that frequency of low wind velocities below 4 m/s account for more than 60% when examining frequency distribution classified by wind velocity for annual winds, issues of actual operating efficiency of wind turbines being very low exist.